well... you get yourself a big hammer.... and beat the :q:q:q:q out of it from the back side. It'll break loose and fall off.
Of course, if you're looking to save the rotor, make sure you use a dead-blow hammer, or a 2lb sledge with some wood between it and the rotor.
When you go to put the rotor back on, or install a new one... Coat the hub face, or back side of the rotor (NOT THE FRICTION SURFACE) with anti-seize. You'll never again have a problem with rotors that won't come off.